Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats

the stream divides into many branches over an alluvial fan which it is building out into the fiord. There is a large area of gravels to the southeast of the expanded ice and gorges cut in them show clearly that they form a thick deposit. One gorge, in particular, now no longer occupied by the stream, has in front of it a well-defined alluvial fan. This abandoned gorge and fan prove clearly that there have been important changes in the drainage along the front of the ice bulb of Butler Glacier. Even more noteworthy in this connection is the fact that the upper surf ace-of the gravels is fluted, by glacial erosion, has a thin moraine veneer, and is crossed by stream channels. At two points there are well-defined stream-bed deposits in anastomasing channels, showing that at a previous stage glacial drainage cut across these gravels at an elevation of 100 feet above the present stream. This stage of drainage was probably at the time when the glacier extended further out to the heads of these channels which are now cut off by the development of the gorge cut in the gravels.
Between this part of the gravel terrace and the sea is a range of low, hummocky hills rising to an elevation of from 100 to 175 feet, which is undoubtedly a stagnant moraine-covered ice block (PL LI, B), entirely disconnected from the Orange Glacier, so far as we could see. In a part of this moraine nearest the glacier, and on the margin of the gorge cut by present-day drainage, buried ice was revealed in the cliff face. The section here from base to top is as follows: Just above the alluvial fan, stratified gravels rise to a height of 25 feet; above this are 50 feet of ice; and, on the top, till moraine 8 feet deep. The moraine-covered ice area extends down to within an eighth of a mile of the sea; but it grows lower toward the sea and encloses in a semi-circle an area where there is no ice. Throughout the surface of this moraine (a, Fig. 9) there is slumping of the soil, cracking of the ground, an abundant outflow of cold water, all clear indications of the presence of buried ice; and at one point, a few hundred feet south of the ice cliff already mentioned, ice was discovered by digging to a depth of about 2 feet at the base of the slumped surface. Even the outer face of the moraine has little vegetation and a tumbled appearance, indicative of recent sliding of the moraine through undermining. Over a large part of the surface of the moraine there is a moss growth; and willow bushes, 5 to 10 years old, are scattered over the entire area; but many are overturned by the sliding of the soil.
The interpretation of these conditions is not difficult. At some earlier stage a series of gravels were deposited here, which continue nearly up to Nunatak Glacier, everywhere showing evidence of overriding by the glacier, which, though it failed to remove them, fluted the surface into terrace form and left upon it a morainic veneer. These gravels, apparently contemporaneous with others in Russell Fiord, antedate the last great advance of the glaciers of the Yakutat Bay region when Nunatak Glacier extended through Nunatak Fiord and even far up Russell Fiord. When the Nunatak Glacier receded past the mouth of the Butler Glacier valley, Butler Glacier expanded in a broad piedmont ice bulb over the entire area between the mountain base and the fiord. This bulb became covered with ablation moraine over a large part of its surface, but a clear ice area existed between the present piedmont area and the stagnant moraine-covered area just described. This clear ice area has completely melted, and by reason of this fact, together with erosion "by the glacial stream, the outer moraine-covered ice area is now completely separated from the glacier which gave it birth. We have here apparently the last stages in the